Optical devices such as waveguides, gratings, and switches, for example, are typically fabricated in layers of silica deposited on silicon. One problem that occurs when employing these materials is that strain birefringence arises because of the large thermal expansion coefficient of silicon relative to silica. As a result of this difference, large compressive strains are produced in the silica layers after the requisite annealing step is performed during the fabrication process. The resulting birefringence caused by the strains produce different propagation constants for the TE and TM waveguide modes. Because the modes have different propagation constants, an optical signal propagating in a device with maximum transmission at a given wavelength is split into two components corresponding to the TE and TM modes. The components are thus spaced apart in wavelength and the difference, which is referred to as the polarization shift, is typically about 0.3 nm. A polarization shift of this magnitude is too large for many applications in which optical devices are employed. For example, frequency routing devices having channel spacings of less than 2 nm are required for long-haul or local area networks. For such purposes the routing device typically should have a polarization shift of less than about 0.1 nm.
It is known that birefringence in silica waveguides is affected by irradiation. For example, copending application Ser. No. 08/396,023 entitled Radiolytic Modification of Birefringence in Silica Planar Waveguide Structures, filed in the U.S. Patent and Trademark Office on the same date as the present application, discloses a method for reducing or eliminating birefringence in silica waveguides by irradiating the waveguides at a wavelength that induces compaction in the waveguide cladding. In addition, Hibino et al, Electron. Lett., 1993, 29, pp. 621-623, indicates that birefringence can be reduced by irradiating the material at a wavelength which is absorbed by photosensitive defects. Hibino irradiated one of the two waveguides in a Mach-Zehnder interferometer to induce birefringence while shielding the other waveguide with a mask. Because only two, relatively widely spaced waveguides were involved, this arrangement was sufficient to direct the radiation to a selected one of the two waveguides.
The irradiation method disclosed by Hibino is not satisfactory when applied to an integrated optical grating rather than a Mach-Zehnder interferometer. Optical gratings include more than two waveguides (and typically include 10-40 waveguides) which are separated by as little as 50 microns. In this case the waveguides cannot be selectively irradiated by masking individual ones of the waveguides. Moreover, since Hibino simply observes that birefringence is affected by radiation, Hibino provides no criteria that can be applied to determine the relative amounts of radiation that should be imparted to the different waveguides in an optical grating to reduce birefringence.